The functions of said useful-data processing unit can also be provided by the service usage computer. In this case the processor of the service usage computer program processes instructions during whose execution the functions of the useful-data processing unit are provided.
The data transfer service relates, for example, to:                the transmission of voice data within the scope of a telephone connection, or        the transmission of video data within the scope of a videoconference.        
Typical functions of the signaling unit relate to providing additional features, in particular for users whose terminals are operated on a private branch exchange.
Typical functions of the useful-data processing unit are:                analog-to-digital conversion or, as the case may be, a digital-to-analog conversion, and        voice data compressing.        
While at upper protocol levels there are connections for transmitting the useful data, when data packets are transmitted in a data packet transmission network, transmission of the data packets at lower protocol levels is connectionless. Signaling for transmission of the useful data has, for example, been established:                for internet protocol telephone services in the protocols of the H.323 protocol family of the ITU-T (International Telecommunication Union-Telecommunication Standardization Sector), in particular in the H.225 and H.245 protocols,        for internet protocol telephone services in the SIP (Session Initiation Protocol) signaling protocol of the IETF (International Engineering Task Force).        